Prostate cancer is the most commonly occurring cancer aside skin cancer in the US, and is the second most common cause of male cancer deaths.
Prostate cancer is classified in four stages: Stage I prostate cancer is found in the prostate only and cannot be felt during a digital rectal exam nor is it visible by imaging. In stage II prostate cancer, the tumor has grown inside the prostate but has not extended beyond it, whereas in stage III, the cancer has spread outside the prostate, but to a minimal extent only. Often, prostate cancer in stage III will have spread only to nearby tissues, such as the seminal vesicles. Finally, in stage IV, the cancer has spread outside the prostate to other tissues, such as the lymph nodes, bones, liver, and/or lungs or brain.
The spectrum of prostate cancers that are progressing despite castrate levels of testosterone includes tumors that have shown varying degrees and durations of response to primary hormone treatment, and clinical manifestations that range from a rising prostate-specific antigen (PSA) alone, a rising PSA with osseous and/or soft-tissue spread, or a predominantly visceral disease pattern.
Currently approved treatment of prostrate cancer includes surgical castration, chemical castration, or a combination of surgical and chemical castration. Removal of the testes, the primary testosterone producing organ, reduces the levels of circulating androgens, to less than 5% of normal levels. This reduction in androgen levels inhibits prostate tumor growth. Although the anti-tumor effects of surgical castration are direct, the anti-tumor effects can be temporary. Surgical castration often leads to clonal selection of androgen-independent prostate tumor cells. This results in re-growth of the prostate tumor in a form that proliferates without testosterone or DHT Stimulation. Chemical castration (also called medical castration) is often substituted for surgical castration, as an initial treatment. Despite its high prevalence, treatment options for men having prostate cancer remain relatively limited and typically depend on the stage of the cancer.
Treatment options include surgical treatments such as radical prostatectomy, in which the prostate is completely removed and radiation, applied through an external beam that directs the dose to the prostate from outside the body or via low-dose radioactive seeds that are implanted within the prostate to kill cancer cells locally. Anti-androgen hormone therapy also is used in the treatment of prostate cancer, either alone or in conjunction with surgery or radiation. Hormone therapy typically aims at blocking the pituitary from producing hormones that stimulate testosterone production by use of castration or administration of hormone analogs and requires that patients have injections of these hormone analogs for protracted periods. Finally, chemotherapeutic approaches have been used to treat advanced prostate cancer, usually as a last resort when other approaches have failed. Since a couple of years, the combination of docetaxel and prednisone was established as the new standard of care for patients who have progressed on androgen deprivation.
None of the treatments described above are curative and prostate cancer being androgen dependent at first, often will progress despite surgical and hormonal-based therapies, and become resistant over time, leading to a cancer type which is called “hormone refractory cancer” or “castration resistant cancer” (CRPC).
Clinical disease manifestations of CRPC are commonly related to bone metastases and may include pain, pathologic fractures, and spinal cord compression, with local recurrences that may be associated with pelvic discomfort, renal dysfunction due to ureteral compression, bladder outlet obstruction, and sexual dysfunction. Further, while bone cancer is the predominant result of CRPC, patients may develop soft-tissue metastases (lymph node(s)) and visceral metastasis in liver, lung, brain, and other organs. Patients with CRPC are minimally responsive to chemotherapy and the majority of patients die due to progressive prostate cancer within 20 months of initiating treatment. Bisphosphonates are commonly used in patients with castrate-resistant prostate cancer who have bone metastases.
It has been shown that prostate tumors remain dormant and clinically undetectable until they begin to secrete angiogenic factors and down-regulate the expression of angiogenic inhibitors. In general, it can be stated that angiogenesis is critical to the genesis of prostate tumors. Therefore, it was not completely surprising that anti-angiogenic agents inhibit prostate cancer cell growth.
In prostate cancer, tumor cells express an abnormal integrin repertoire and are surrounded by a markedly aberrant extracellular matrix (ECM). These changes have profound consequences, given the ability of each integrin to regulate specific cell functions. Expression of β3 and β1 subunits activates specific signaling pathways and support distinct cancer cell functions. β3 is uniquely required in cancer cells for increasing cdc2 levels as well as cdc2 kinase activity. These effects are specific for β3 and are not observed for β6. Up-regulation of β3 and β6 integrin variants has been described. Zheng et al. (Cancer Research 1999; 59, 1655-1664) used human prostate cancer cells isolated from sixteen surgical specimens, to show that these cells express αvβ3, whereas normal prostate epithelial cells do not. Similarly, αvβ6 was found to be expressed in adenocarcinoma (Li et al.; Molecular and Cellular Biology 2007; 27, 4444).
The use of integrin inhibitors is likely to affect both cancer cell survival and angiogenesis since integrins are expressed by tumor cells as well as by endothelial cells. Although it is hard to discriminate between an effect on tumor growth and an effect on angiogenesis, a maximal response of these inhibitors can be predicted when the targeted integrin is expressed by both tumor and endothelial cells.
Bone is the most frequent metastatic site for prostate cancer. Bisanz et al. (Molecular Therapy 2005; 12, 634-643) illustrate a positive role for alpha-v integrins on prostate tumor survival in the bone. Analysis of human prostate cancer bone xenografts shows that intratumoral administration of liposome encapsulated human alpha-v siRNAs significantly inhibits the growth of PC3 tumors in bone and increases apoptosis of prostate tumor cells. Further studies (McCabe et al., Oncogene 2007; 26, 6238-6243) demonstrate that αvβ3 integrin activation on tumor cells is essential for the recognition of key bone specific matrix proteins. These data suggest that the αvβ3 integrin modulates prostate cancer growth in distant metastasis.
Since integrins mediate the interactions between tumor cells and bone microenvironment and facilitate growth in bone, a potential application of the use of integrin inhibitors is to prevent prostate cancer bone lesions. These lesions are osteoblastic and/or osteolytic and are frequently detected in prostate cancer patients (over 80% of prostate cancer patients have established bone metastasis at autopsy).
A recent study has shown that the αvβ3 integrin promotes bone gain mediated by prostate cancer cells that metastasize to the bone and point to αvβ3 as a potential therapeutic target to block prostate cancer osteoblastic lesions. Immunohistochemical analysis has demonstrated the presence of αv integrin in a large proportion of human prostate cancer tissues samples.
These and other results suggest that anti-integrin agents may have both direct and indirect antitumor activity. But there are only few clinical trials reporting that peptide or non-peptide integrin inhibitors are effective agents in prostate cancer therapy.
Therefore, there is a need to provide a potent anti-integrin agent for use in the therapy of prostate cancer, especially castration-resistant prostate cancer developing bone metastases.